One of the major features of LTE-Advanced is Carrier Aggregation, which allows mobile network operators to combine a number of separate LTE carriers. This enables them to increase the peak user data rates and overall capacity of their networks and to exploit fragmented spectrum allocations. I introduced the fundamentals of Carrier Aggregation in a previous post on LTE Carrier Aggregation comes to fruition and I discussed the first commercial deployment in SK Telecom leads the way with LTE-Advanced.

In principle, Carrier Aggregation can be applied to either the FDD or TDD variants of LTE and it can be used to combine carriers whether or not they are contiguous or even in the same frequency band. Ultimately the aim is for Carrier Aggregation to combine up to five separate LTE carriers, each of up to 20MHz. However, early implementations are rather more limited than this. For example, the current 3GPP standards (up to Release 11) support the aggregation of just two carriers and only in a subset of the full range of LTE operating frequencies. Nonetheless, even these early implementations have significant benefits. SK Telecom identified Carrier Aggregation as the highlight of its LTE-Advanced launch in June 2013, by enabling it to combine 10MHz carriers in its 800MHz and 1.8GHz bands to achieve peak downlink user data rates of 150Mbps.

Carrier Aggregation has much more to offer and it continues to be a significant area of work for 3GPP, equipment manufacturers and network operators. Over the coming years there will be a number of important developments, including:

• Increasing the number of component carriers and the total bandwidth supported in both the downlink and the uplink.
• Supporting a greater number of frequency bands and combinations of frequency bands.
• Using Carrier Aggregation between cells to enhance the support of small cells and Heterogeneous Networks (HetNets).
• Enabling flexible aggregation of FDD and TDD LTE carriers.
• Supporting LTE Carrier Aggregation between licensed and unlicensed spectrum.

The following figure shows major steps in the evolution of LTE Carrier Aggregation:

3GPP Release 10 (frozen 2011) introduced a limited set of Carrier Aggregation options, as summarised in the table below:

Release 10 set the precedent for LTE Carrier Aggregation and addressed some specific early requirements, but it was just the start. The LTE system is able to operate in more than 40 different spectrum bands, to accommodate diverse frequency allocations in different parts of the world. Each potential combination of bands needs to be studied on its own merits to identify any potential issues with Carrier Aggregation, such as intermodulation effects and implications for transceiver design.

3GPP Release 11 (frozen 2013) adds significantly more band options to the existing operating modes and also introduces the capability for intra-band non-contiguous operation in FDD and TDD, as summarised in the following table:

3GPP Release 12 (to be frozen 2014) will add further frequency bands and band combinations to the existing carrier aggregation modes. It will also introduce two new modes, enabling three downlink/one uplink carriers as well as two non-contiguous uplink carriers.

Ahead of the completion of 3GPP Release 12, Samsung demonstrated three carrier channel aggregation at Mobile World Congress (MWC) 2014. The demonstration combined three 20MHz carriers to achieve a peak downlink user data rate of 450Mbps.

Beyond the extension of current Carrier Aggregation modes to more channels, more bands and more combinations of bands, 3GPP is also investigating a number of other potentially valuable developments:

• Aggregation of carriers across cells.
• Aggregation of TDD carriers with FDD carriers.
• Aggregation of carriers across licensed and unlicensed bands.

Aggregation of carriers across cells

In principle Carrier Aggregation can be applied across carriers hosted by different cells. There are already features in Release 11 to support this if there is sufficient coordination between the cells, but further developments are underway to enable Carrier Aggregation across a mixture of macro cells and small cells when they have non-ideal backhaul. This will have significant benefits in HetNets by enabling network operators to provide robust mobility management on their macrocell networks, while providing the capacity and performance benefits of Carrier Aggregation in local areas where small cell coverage is available.

Aggregation of TDD carriers with FDD carriers

Many network operators have both TDD spectrum and FDD spectrum. For example, Vodafone UK has 25MHz of unpaired spectrum (suitable for TDD) in the 2.6GHz band, in addition to paired spectrum (suitable for FDD) at lower frequencies. The ability to aggregate the two types of spectrum has a number of potential benefits. For example, TDD spectrum could be used to supplement FDD spectrum to provide additional throughput and capacity on the downlink. Conversely, FDD spectrum, which is generally at lower frequencies than TDD spectrum, could be used to achieve greater range on a TDD uplink, which is often the limiting factor for TDD coverage.

In 2013, as part of the Release 12 programme, 3GPP established a new work item on joint operation of the TDD and FDD LTE modes, including Carrier Aggregation between them. Among other things, this work is defining a number of deployment scenarios for TDD/FDD operation, identifying technical requirements and potential solutions for joint operation, and defining a generic framework for Carrier Aggregation between the two modes. The aim is to provide flexible aggregation options, which allow either TDD or FDD to act as the primary carrier in the aggregated connection and to carry the associated control signalling. The findings of the work are being compiled in 3GPP TR 36.847

Even before the completion of standards on this, manufacturers and network operators are already forging ahead with TDD/FDD aggregation. For example, at MWC 2014:

• Vodafone and Huawei demonstrated FDD/TDD Carrier Aggregation using 3 FDD carriers and 1 TDD carrier to achieve a single user peak downlink data rate of over 500Mbps.
• Nokia Solutions and Networks (NSN) with Korea Telecom (KT) and SK Telecom (SKT) demonstrated TDD/FDD Carrier Aggregation and highlighted the benefits to both TDD LTE and FDD LTE networks, including enhanced coverage for TD-LTE by aggregating lower frequency FDD spectrum.

Aggregation of licensed and non-licensed spectrum

Today’s LTE networks are deployed in licensed spectrum bands. However, ever-increasing demands for capacity will require network operators to exploit all possible spectrum resources available to them. Among these are large tranches of unlicensed spectrum, including roughly 500MHz available in many regions of the world in the 5GHz band. This spectrum is not dedicated to individual network operators in the same way as licensed spectrum, and it is used by other technologies, such as WiFi, as a result of which its occupancy is somewhat variable and unpredictable. Nonetheless, this is a valuable resource.

One possible approach to harnessing unlicensed spectrum is to use Carrier Aggregation. Connections could be anchored in reliable, licensed spectrum, to provide reliable mobility management, while unlicensed spectrum is used opportunistically to boost throughput and capacity where and when it is available.

The wide bandwidth, low power and limited propagation of the 5GHz band make it well suited to small, high capacity cells set in a backdrop of wide area coverage provided by lower frequency licensed band macrocells. Dynamic Channel Allocation could be used to minimise the impact between LTE and WiFi in unlicensed bands. Carrier Aggregation for unlicensed bands is a candidate for 3GPP standards beyond Release 12.

Carrier Aggregation will continue to receive the attention of 3GPP, equipment manufacturers and network operators for some time to come and it will be a valuable asset as they race to deliver the capacity and performance required over the next ten years.